As one of line plotting algorithms for plotting lines whose coordinates are given on a display screen on which pixels are arranged, there is a method called DiamondeExit Rule. When lines are plotted according to this algorithm, even in a continuous line plotting, their joints can be connected without any gap and also there occurs no second plotting of pixels. Conventionally, when a DiamondExit Rule is not used, lines are sometimes broken. Therefore, it is preferable to apply a DiamondExit Rule when plotting lines.
However, when applying this rule, care is required when plotting a line whose length is less than 1. This is because there are a case where a line is completely vanished (no pixel is plotted) and a case where only one pixel is plotted, depending on the positions of the starting point and ending point of a line.
When applying a DiamondExit Rule, a mechanism for determining how to give the starting and ending points of a line whose length is less than 1 about all thinkable patterns is necessary. However, when attempting to realize logic for determining how to plot about all the patterns by a digital circuit, its circuit scale becomes large and also its process speed slows down. Therefore, a Diamond Exit Rule which can be applied when plotting a line whose length is less than 1 and be realized by simpler logic is desired.